A high electron mobility transistors (HEMT) using, for example, GaN has been used as semiconductor devices using compound semiconductors, specifically as high-power/high-frequency elements. FIG. 5 illustrates an outline of a cross-sectional structure of a HEMT device (semiconductor device) 10 using a nitride semiconductor. In FIG. 5, on a substrate 11, an electron transit layer 12 and a barrier layer 13 are sequentially formed by epitaxial growth. Herein, for example, the electron transit layer 12 is made of semi-insulating (undoped) GaN, and the barrier layer 13 is made of semi-insulating (undoped) AlGaN (precisely semi-insulating (undoped) AlxGa1-xN, where x is about 0.20). In this structure, on a side of the electron transit layer 12 in the vicinity of an interface (hetero interface) of the electron transit layer 12 and the barrier layer 13, a two-dimensional electron gas layer 14 (shown by a broken line in FIG. 5) that is to be an electrically conductive layer is formed in parallel with the hetero interface by a piezoelectric effect. According to the two-dimensional electron gas layer 14, a current flow between a source electrode 15 and a drain electrode 16, and a channel configured by the two-dimensional electron gas is switched on or off according to a voltage applied to a gate electrode 17 that is to be a Schottky electrode. At this time, since the speed (mobility) of electrons in the two-dimensional electron gas is very high, a high-speed operation is possible. Further, since GaN has a band gap wider than of that of GaAs or the like, the HEMT device 10 has a high pressure resistance and thus is capable of a high-power operation.
In the HEMT device 10, in order to increase the mobility in the two-dimensional electron gas layer 14 and obtain high conductance, it is required that the interface (hetero interface) of the electron transit layer 12 and the barrier layer 13 is abrupt, that is, a composition variation between the electron transit layer 12 and the barrier layer 13 is abrupt at the interface. The electron transit layer 12 and the barrier layer 13 are consecutively formed by, for example, a metal organic chemical vapor deposition (MOCVD) method, a molecular beam epitaxy (MBE), or the like.
In order to obtain a HEMT device having high mobility, for example, JP-A-2004-200711 discloses a technology of inserting a spacer layer, which has high aluminum composition and a wide band gap, in the vicinity of a hetero interface. FIG. 6 is a cross-sectional view illustrating a cross-sectional structure of a HEMT device (semiconductor device) 30 disclosed in JP-A-2004-200711. Here, between an electron transit layer (GaN layer) 12 and a barrier layer (AlGaN layer) 13, an AlN spacer layer 20 is as thin as one to four molecular layers (about 0.25 nm to 1 nm) is inserted. It is possible to widen the band gap at a position of the spacer layer 20 and to improve abruptness in a band structure, thereby substantially improving abruptness at the hetero interface. According to this technology, it is possible to increase the mobility at the two-dimensional electron gas layer 14 and to obtain a HEMT device having high conductance.